The present invention relates generally to program start and dimmable electronic ballasts for gas discharge lamps. More particularly, the present invention relates to an electronic ballast with integrated, frequency independent and controllable filament voltage drive circuitry.
Filament voltage control is highly important for fluorescent lamp life. A good filament control circuit should first have sufficient preheat capabilities prior to lamp ignition. The preheat voltage should not change with the number of lamps connected to the ballast.
Second, the filament control circuit should function to cut off the filament heating after ignition of the lamp if the lamp operates in a high current stage to save energy and improve the lamp efficiency.
Third, the filament control circuit should provide proper filament heating during a dimming phase according to the lamp requirements.